The telecommunication international standardization body, 3GPP (3rd Generation Partnership Project), has completed the standardization of LTE (Long Term Evolution), which is the 3.9th generation telecommunication system, and currently is making progress in LTE-Advanced (LTE-A) adopting the LTE as the 4th generation telecommunication system. In the LTE-A, a relay technology of relaying radio signals using a relay station (Relay Node: RN) to expand a coverage and improve a capacity has been examined, as in NPL 1.
The relay technology in the LTE-A will be described in brief with reference to FIG. 11. FIG. 11 is a diagram illustrating a communication system using a relay technology. In FIG. 11 an eNB (evolved Node B) indicates a base station, an RN (Relay Node) indicates a relay station, and a UE (User Equipment) indicates a terminal. In FIG. 11, a UE1 indicates a terminal connected to the eNB and a UE2 indicates a terminal connected to the RN.
In the LTE-A, it has been examined whether a separate cell ID is also allocated to an RN, as in an eNB. Then, an RN can create one cell (relay cell), as in a cell (macro cell) created by an eNB. In the LTE-A, such a relay technology is called Type 1 Relay. The eNB is connected to a network in a wired communication manner and the RN is connected to the eNB in a wireless communication manner. A communication link connecting the RN and eNB to each other is called a backhaul link.
On the other hand, a communication link connecting the eNB and UE or the RN and UE is called an access link. In a downlink (DL), the RN receives a signal from the eNB using the backhaul link and transmits a signal to the UE2 using the access link of the RN. In an uplink, the RN receives a signal from the UE2 using the access link of the RN and transmits a signal to the eNB using the backhaul link. A relay technology using the backhaul link and the access link with the same frequency band is called in-band Relay in the LTE-A. In In-band Relay, when the RN transmits and receives signals at the same timing, interference may occur since the transmitted signal comes around to the received signal. Therefore, the RN may not transmit and receive signals at the same timing. Accordingly, in the LTE-A, a relay scheme of allocating timings of the backhaul link and the access link of the RN in a sub-frame unit has been examined.
Further, in the LTE-A, a technology of reducing power consumption in an eNB has been examined against the background of recent environmental problems. NPL 2 examines a method of reducing power consumption in an eNB by providing a time at which a signal is not transmitted from an eNB using a downlink. In an RN, it is also necessary to reduce power consumption due to the same reason. In addition to this, there is the following reason to reduce power consumption of an RN. That is, since there is a probability that the RN is driven by a battery, it is necessary to reduce the power consumption of the RN. From the viewpoint of the expansion of a coverage which is one of the objects of an RN, in regard to the installation place of an RN, it is considered that an RN is installed to relay radio waves of an eNB to a UE to which the radio waves do not arrive from the eNB. A mountain area and a vast plain area are examples of an area to which the radio waves do not arrive from the eNB. In this installation area, it is sometimes difficult to provide a power cable connected to the RAN. Therefore, it can be considered that the RN is driven by a battery.
Since a UE is originally driven by a battery, a discontinuous reception (DRX) technology of reducing reception power consumption of a UE is adopted in the LTE, as in NPL 3. In the DRX technology, the reception power consumption is designed to be reduced by providing a time at which a PDCCH (Physical Downlink Control Channel), which is a control channel of the downlink along which a signal is transmitted from an eNB, is not monitored in a UE.
The DRX technology of a UE in the LTE as a conventional DRX technology will be described in brief with reference to FIG. 12. FIG. 12 is a diagram illustrating a DRX technology of a UE in the LTE. In FIG. 12, an upper part shows DL transmission sub-frames Tx in an eNB and a lower part shows DL reception sub-frames Rx in a UE. Further, a middle part shows DRX Cycle and a PDCCH monitoring period in the UE.
First, the eNB notifies the UE, to which the DRX technology is applied, of a parameter which determines a DRX interval. Examples of the parameter which determines the DRX interval include DRX Cycle which is a repetition period of DRX and “On Duration Timer” which is a period in which the UE monitors the PDCCH transmitted by the eNB in DRX Cycle. Since the UE is notified of the parameter using an RRC signaling, a relatively long time is necessary until setting of the DRX is changed.
The UE performs the DRX based on the notified parameter. As shown in FIG. 12, the UE determines a PDCCH monitoring period (indicated by “On Duration” in FIG. 12) based on the notified parameter. The UE monitors the PDCCH transmitted from the eNB in the PDCCH monitoring period. In a period other than the PDCCH monitoring period, the UE does not monitor the PDCCH and interrupts a reception process. Thus, the reception power consumption can be reduced.